Proton Hop Paving the Way for Hydroxyl Migration: Theoretical Elucidation of Fluxionality in Transition-Metal Oxide Clusters

The reactions of chemisorbed water on W3O6 and Mo3O6 clusters have been investigated to explore the phenomenon of fluxionality in transitionmetal oxide clusters. The net observed phenomenon here is a hydroxyl migration. However, mechanistic studies using electronic structure theory reveal that the hydroxyl migration occurs by a synergistic pathway led by a proton hop and assisted by an interconversion between a bridging oxygen and a terminal oxygen. The proton hop provides access to two isomers, which differ in the relative position and orientation of the hydroxyl groups, thereby generating the scope for an enhanced catalytic activity in important processes such as hydrogen evolution from water. SECTION Dynamics, Clusters, Excited States C atalytic applications of transition-metal oxides (TMOs) are innumerable. Small clusters of TMOs have been used as model systems to investigate the reactivity of low-coordinate and defective surface sites on thesematerials. For instance, hydrogen evolution fromwater and activation of inert chemical bonds (e.g., C-H) are important areas where these systemshave been investigated recently in our group. Potential catalytic applications have driven the scientific community to probe these TMOs for unraveling interesting chemical features that could further assist in opening newer avenues in research. The electronic structures of the TMO clusters have been investigated by many research groups. A significant number of contemporary theoretical and experimental studies on these TMO clusters have been focused on their reactions with small molecules. A deeper understanding of the physical and chemical factors affecting the reactivity of the TMO clusters is extremely useful in furthering their catalytic applications. One suchmajor factor, fluxionality, is a keyaspect in cluster science. Historically, it is well-known that fluxionality can significantly affect the reaction sites in a molecule. Proper exploitation of fluxionality can therefore have a significant impact on the alteration of the reaction site of a molecule and hence can be invaluable in catalysis, a point strikingly illustrated by the dexterous utility of fluxionality in purely metallic clusters. However, to the best of our knowledge, there have not been many studies on the fluxionality of TMO clusters. While Dixon and co-workers in 2006 have reported that larger rings of certain TMOclusters exhibit structural fluxionality, the effect of chemical reactivity on fluxionality and the scope for catalysis have hitherto not been explored. The first step toward utilizing fluxionality in TMO clusters involves the elucidation of structural fluxionality in TMO clusters when they react with small molecules. In this letter, we theoretically demonstrate the phenomenon of protonassisted fluxionality in TMOclusters.On thebasis ofour current interest in the catalytic applications of molybdenum and tungsten oxide clusters toward hydrogen evolution from water, chemisorbed water on the surface of W3O6 andMo3O6 clusters has been chosen to study this phenomenon of proton-assisted fluxionality. The “proton hop” effectively results in a hydroxide group migration, whereas the actual process occurring here is a proton migration resulting in the swapping of positions between a terminal oxygen and a bridging oxygen. Such a chemical rearrangement results in two isomers which differ in the position as well as relative orientation of the hydroxide groups. This creates an alteration in the reactive site of the TMO clusters before and after the proton hop, thereby influencing their catalytic activity. The equivalence of different types of oxygens in the TMO clusters and the fact that oxygens (one of which belongs to chemisorbed water and the rest to the TMO clusters) and hydrogens (of chemisorbedwater) are the atomsmost actively involved in the fluxionality of the TMO clusters warrant a simplified notation for the atoms involved to avoid confusion in the following discussion. In this study, the oxygen of water is denoted as O (inset, Figure 1). Irrespective of the TMO cluster under study (W3O6 or Mo3O6 ), it is observed that water dissociates as H and OH, and the H adds to a bridging oxygenattached toWandW (orMo andMo). The remainingmetal center (tungstenormolybdenum) is denoted asW. Received Date: August 25, 2010 Accepted Date: September 29, 2010